Studies will investigate the role of the renin-angiotensin system in oral contraceptive-induced hypertension in rabbits, by examining the plasma levels of plasma renin activity, plasma renin concentration, and plasma renin substrate concentration, and by investigating the effect of a competitive antagonist of angiotensin II on mean arterial pressure. Experiments also will examine the effects of a synthetic estrogen (mestranol) and a synthetic progestin (norethynodrel) on the arterial pressure in rats. Additional studies will determine the effect of long-term infusions of a competitive antagonist of angiotensin II on the development and maintenance of hypertension due to renal artery stenosis in rabbits. The hemodynamic and plasma renin activity changes will be examined in rabbits 6 to 8 hours after renal artery stenosis. The pressor responses to angiotensin II and to vasopressin will be determined in pre-hypertensive rabbits with renal artery stenosis. The effects of renal artery stenosis on plasma volume, extracellular fluid volume, and total body water will be determined in pre-hypertensive and hypertensive rabbits with renal artery stenosis. Studies will be continued on the plasma concentration of a non-renin-angiotensin pressor substance in the plasma of renal hypertensive and normal rabbits.